1. Introduction
This invention relates to electroless metal deposition and more particularly, to catalytic compositions useful therefor.
2. Description of the Prior Art
Electroless metal deposition refers to the chemical deposition of a metal on a conductive, non-conductive, or semi-conductive substrate in the absence of an external electric source.
Electroless deposition is used for many purposes, for example, in the manufacture of printed circuit boards where, in one method, an electroless metal, typically copper, is deposited on a dielectric substrate either as a uniform surface coating or in a predetermined pattern. This initial electroless deposit is usually thin and may be further built up by electroplating or may be deposited directly to full thickness.
The substrate over which an electroless metal deposit is formed is most often a plastic panel which may have a metal foil such as copper laminated to one or both of its surfaces, for example, with adhesive, to form a metal clad substrate. Where both surfaces of the substrate are to be used, connections are typically provided therebetween by means of holes through the panel at appropriate locations, the walls of these holes being made conductive with the electroless coating.
The electroless deposition of a metal on either a metallic or non-metallic substrate usually requires pretreatment or sensitization of the substrate to render it catalytic to the reception of such a deposit. Various methods have evolved over the years employing particular sensitizing compositions.
An early method of sensitization involves plural baths wherein a substrate is subjected first to immersion in an acidic stannous chloride solution and, following water rinsing, immersion in an acidic palladium chloride solution whereby the palladium chloride is reduced to a catalytic layer of palladium metal. This method has severe limitations because the palladium chloride solution, in contact with the copper layer of a copper clad substrate, forms an immersion deposit which is loosely adhered to the copper. This is wasteful of palladium and interferes with the bond between the copper cladding and a subsequently deposited electroless metal layer.
A major advance in the art of electroless metal deposition was achieved by the process of U.S. Pat. No. 3,011,920 incorporated herein by reference. According to said patent, a substrate was catalyzed by treatment with a liquid which contained a colloidal dispersion of a catalytic metal formed by the admixture of catalytic metal ions and stannous ions in an amount in excess of the catalytic metal ions in an acidic aqueous solution. By this method, the immersion coating formed by contact of palladium ions with copper cladding was eliminated as the catalytic metal is not in an ionic form, but rather, is in colloidal form, where the colloid is believed to be in an insolubilized reduced form of the catalytic metal. Other advantages were achieved with the invention of U.S. Pat. No. 3,011,920 such as decreased cost resulting from reduction in the number of steps in an overall plating process and reduction in the amount of catalytic metal consumed, improved coating, improved bond strength between coating and substrate, greater reliability and other improvements as enumerated in said patent.
For purposes of economy in shipping and storage, it is desirable to provide a catalyst in dry form which can be redispersed by simple admixture with an aqueous acidic solution. With reference to the catalyts of U.S. Pat. No. 3,011,920, it was believed that the catalysts thereof could not be dried and redispersed. This belief was due in part to stability tests where catalyst was left exposed in an open container with a large surface area, such as a petri dish. In these tests, it was found that with standing and evaporation of the liquid components of the catalyst, the catalyst would coagulate prior to complete drying and once coagulated, could not be redispersed.
In the prior art, one attempt at production of a dry catalyst material is reported in U.S. Pat. No. 3,672,923. The catalyst of said patent, prior to drying, is reported to be made by the admixture of catalytic metal ions and a reducing agent therefor such as stannous ions in an aqueous acidic medium under specific conditions such that, rather than forming a colloidal catalyst as in U.S. Pat. No. 3,011,920, a complex optically clear non-colloidal "true solution" catalyst of unreduced catalytic metal ions is formed. It is reported that because this catalyst is a true solution complex, it can be dried to powder form whereas a colloidal catalyst could not be similarly dried. A typical reported method for making a complex catalyst, as opposed to a colloidal catalyst, and drying the same is set forth in Example 3 of the patent wherein stannous chloride is dissolved in 37% hydrochloric acid, diluted and admixed with palladium chloride. The mixture is heated to 85.degree. C for 20 minutes, then boiled for 1.5 hours, cooled and vacuum evaporated to dryness. It is reported that this procedure forms a true solution complex catalyst.